The methods and steps employed in the processing of tea leaves from tea plants during decaffeination into decaffeinated tea products have a significant influence on the quality of the resulting tea products, in particular in terms of flavor and/or intensity. The quality is in particular determined by the content of tea catechins (which might make up 30% of the dry weight of tea leaves in green tea), theaflavin derivatives, such as theaflavin-3-gallate, and/or other polyphenols. Thus, the method employed for processing tea leaves into tea products, in particular into decaffeinated tea can be critically important to the commercial viability, success or acceptance of the tea products. Another factor that is important for the commercial viability of such products is in how far the by-products of the decaffeination, in particular the caffeine, can be recovered. Generally, caffeine constitutes about 3% of the dry weight of tea leaves from tea plants, translating to between 30 mg and 90 mg per 8 oz (250 ml) cup depending on type.
Coffee and tea are both alkaloid containing substances. The main alkaloid they contain is the xanthine alkaloid caffeine, (C8H10N4O2.1,3,7-Trimethyl-1H-purine-2,6(3H,7H)-dione), which is moderately soluble in water at room temperature (2 g/100 ml), but very soluble in boiling water (66 g/100 ml). The radius of caffeine molecule is 3.76 Å with the observed hydrodynamic diameter being ˜1 nm in caffeine solution.
However, the substances that make tea and coffee attractive to drink differ, apart from the caffeine, substantially. The flavor of tea is primarily determined by water soluble components that can be found in the freshly harvested tea leaves as well as the dried and/or oxidized version thereof. The flavor of coffee on the other hand is primarily determined by the roasting process the coffee is subjected to after the drying of the beans.
Tea or coffee beverages are typically obtained by brewing tea leaves or coffee powder (made from roasted coffee beans) in hot water. The water temperature for brewing tea lies typically between 70 and 100 degrees Celsius, depending on the quality of the tea and the taste desired. Coffee is typically brewed at 100 degrees Celsius or above 100 degrees Celsius and frequently pressure is applied during the extraction of the water soluble components.
Thus, while in tea there are many desirable water soluble components in the fresh, dried and/or oxidized (but unroasted) tea leaf, in coffee most of the water soluble components are only generated during the roasting process, while the green, unroasted, coffee bean only contains as water soluble components, caffeine and a few non-water soluble flavor precursors, which become water soluble during the roasting process.
Accordingly, there is a prominent water based coffee decaffeination process, namely, the “Swiss Water Process.” In this process the caffeine is extracted from green, unroasted coffee beans using water. The caffeine is removed from the mixture using active carbon. The decaffeinated coffee beans are dried and roasted. The roasting confers the flavor to the coffee. Generally the caffeine is irreversibly attached to an active carbon.
A typical tea processing procedure begins with the harvest of the tea leaves followed by fermenting and/or drying them. After picking, the leaves of, e.g., Camelia sinensis soon begin to wilt and oxidize unless they are immediately dried. The leaves turn progressively darker as their chlorophyll breaks down and tannins are released. This enzymatic oxidation process is caused by the plant's intracellular enzymes and causes the tea to darken. In tea processing, the darkening is stopped at a predetermined stage by heating, which deactivates the enzymes responsible. In the production of black teas, the halting of oxidation by heating is carried out simultaneously with the drying.
A decaffeination process leaves generally some caffeine in the leaf. By law, tea labeled as “decaffeinated” must, in most countries, have less than 2.5 percent of its original caffeine level, which usually equates to less than 2 mg per cup.
Tea decaffeination often uses processes analogous to the “direct method” of coffee decaffeination.
In the direct method, the tea leaves are first steamed for 30 minutes and then repeatedly rinsed with either dichloromethane or ethyl acetate for several hours. The solvent is then drained away and the leaves are steamed for a time sufficient to remove residual solvent. Sometimes teas that are decaffeinated using ethyl acetate are referred to as “naturally processed” because ethyl acetate can be derived from various fruits or vegetables. However, mostly any ethyl acetate used for decaffeination is synthetic, not natural. In addition, the ethyl acetate based process removes many of the aromatic components of tea that are appreciated by consumers. Accordingly, the dichloromethane based decaffeination process is the primary process used for decaffeinating tea.
Decaffeinated teas that are labeled “organic” are often decaffeinated via a carbon dioxide (CO2) based method. Tea decaffeinated using a CO2 method retains 92 percent of its polyphenols (antioxidants) compared to tea decaffeinated using the ethyl acetate process, which only retains 18 percent. In the CO2 decaffeination method, the CO2 used for decaffeination is filtered and recycled at a rate of around 99%. In this method, the tea leaves are essentially “pressure cooked” with CO2. At high pressures and high temperatures, CO2 reaches a supercritical state. The CO2 becomes a solvent that, with its small, nonpolar molecules, attracts the small caffeine molecules. Since “flavor molecules” such as polyphenols are generally larger, they remain intact, which is why this process retains the flavor of the tea well. After passing through the tea leaves, the caffeine-laden CO2 is filtered to remove the caffeine, and then recycled for further use in decaffeination. U.S. Pat. Nos. 4,167,589 and 4,976,979 describe CO2 based decaffeination processes. These patents, as well as any other patent and non-patent publications referred to herein are incorporated herein by reference in their entirety.
While CO2 based tea decaffeination processes can be considered as natural, they require expensive equipment and complex process controls.
Both organic solvents generally used for extracting caffeine from tea, namely ethyl acetate and dichloromethane, have a medium polarity that permits dissolution of the caffeine, but minimizes concomitant extraction of polar, more water soluble taste and flavor components. Supercritical carbon dioxide in its liquid state is a moderately non-polar to medium polar solvent and thus provides certain selectivity for extracting caffeine.
When water is used as a natural solvent, the conditions applied for removing the caffeine from tea leaves, are relatively close to those applied in preparing tea as a beverage and thus can be expected to lead to an concomitant extraction of those water soluble components other than caffeine that majorly contribute to the taste of the final tea product. Thus, one of the challenges of a water based method for extracting tea leaves is to obtain a product that, nonetheless, contains desirable concentrations of water-soluble components that contribute to the taste profile of the final product.
Thus, there remains a need for a “natural” decaffeination process for tea, in particular a water-based process that does not involve chemical extraction of caffeine and that uses relatively simple equipment and/or process controls. There is also a need for recovering the extracted caffeine at high levels. The present invention is directed at achieving this/these and/or other goals and/or objectives.